The present invention relates to a control system for an injection-molding machine. More particularly this invention concerns such a control system capable of establishing three different pressure levels in the ram operating such a machine.
An injection-molding machine has a plastifying screw rotatable and reciprocal in a housing. One end of the housing is formed with an injection head having an orifice through which plastified synthetic-resin material is ejected under pressure into a mold and the other end of the housing is provided with a laterally opening filling orifice to which granulated synthetic-resin material is chargeable.
During a normal molding cycle the granulated and unplastified synthetic-resin material is loaded into the back end of the injection-molding machine as the worm is rotated and the housing of the machine is heated. Thus the rotating worm acts as an auger and tries to force the material forwardly in the housing. Since at the beginning of the operation the forward end of the housing is closed, however, this has the effect of slowly driving the worm backwardly against the force of a ram. During this part of the cycle a relatively low pressure is maintained in the working chamber of the ram so that the ram can be forced backwardly but the plastified mass is maintained under pressure.
Once the injection-molding machine chamber is filled the pressure is greatly increased to a so-called injection pressure in the ram operating the worm. This forces the worm forward, piston-fashion to extrude the plastified synthetic-resin material from the end of the machine.
It has been found that for best molding results it is essential in the last portion of the forward advance of the worm to change the pressure from the ejecting pressure to a so-called compacting or holding pressure. It is essential for best molding results that the injecting pressure and the holding pressure be maintained within very narrow ranges, and that the change-over from the one pressure to the other be effected at a very carefully controlled and ascertained instant.
In order to operate the ram of the injection-molding machine at these three different pressure levels it is necessary to provide a programming device normally incorporating a timer operating by means of limit means carried on the injection-molding machine. All such programming arrangements have proven themselves unsatisfactory in use in that they were unable to react with sufficient speed to change over, for instance, from the injection to holding pressure at just the right instant, or the various ranges changed with time or temperature due to sensitivity of the electronic components.